feat(endpoints): add preferred endpoint setting

Added a new setting to remember the user's preferred endpoint. When both USB and
BLE are connected, the preferred endpoint will be used.

Added a new behavior to control this setting. It supports commands:

    &end END_USB - Prefer USB output
    &end END_BLE - Prefer BLE output
    &end END_TOG - Toggle between USB and BLE

app/include/dt-bindings/zmk/endpoints.h

@@ -0,0 +1,13 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define ENDPOINT_TOGGLE_CMD 0
+#define ENDPOINT_USB_CMD 1
+#define ENDPOINT_BLE_CMD 2
+
+#define END_TOG ENDPOINT_TOGGLE_CMD
+#define END_USB ENDPOINT_USB_CMD
+#define END_BLE ENDPOINT_BLE_CMD